Conventionally, PET (positron emission tomography) measurement, fluorescent imaging, MRI (magnetic resonance imaging), etc., have been used as methods to make a non-invasive diagnosis for a living body. In such methods, molecules that are present in a living body, such as those of proteins, antibodies, peptides and sugar chains, are labeled using, for example, radioactive labels, and such labels are detected to form an image of the distribution or the like of such molecules in a living body. Using the image thus formed, it is possible to diagnose a disease in the living body.
However, it is not easy with many of these methods to perform such a labeling procedure. In the case of, for example, PET measurement that uses a labeled compound that is labeled with 11C, which has a short half-life, it is necessary to perform the process from label production to PET measurement in a short period of time. Accordingly, it is necessary to install a cyclotron, an automated synthesizer and like devices in medical facilities where PET measurement is performed and to produce such a label for every PET measurement (see, for example, Patent Document 1). In addition, such methods are also problematic in that, e.g., the rate of reaction for labeling a biopolymer using a label is low and the reaction yield is low, thereby making it difficult to label unstable biomolecules.
It is known that a compound that has a hexatriene-β-keto structure undergoes cyclization due to an azaelectrocylic reaction with, for example, an amino group of a lysine residue (see, for example, Non-Patent Documents 1, 2 and 3). For example, a compound having a hexatriene-β-keto structure as represented by the formula below promptly reacts with the lysine residue of phospholipase A2 (PLA2, forms a Schiff base and then undergoes cyclization.

However, no compound that has the aforementioned hexatriene-β-keto structure and that is advantageous for use in the aforementioned labeling procedure is known.    Patent Document 1: JP 2005-154374A    Non-Patent Document 1: Tanaka et al., Tetrahedron Lett., 1998, Vol. 39, pp. 1185-1188.    Non-Patent document 2: Tanaka et al., Tetrahedron, 1999, Vol. 55, pp. 1657-1686    Non-Patent document 3: Tanaka et al., J. of Synth. Org. Chem. Japan, 1999, Vol. 57, pp. 876-887